Polysiloxane compositions are used in various industries because of their excellence in heat resistance, cold resistance, weather resistance, light resistance, chemical stability, electrical characteristics, flame retardancy, water resistance, transparency, colorability, anti-adhesive properties, and anti-corrosive properties. In particular, compositions containing polyhedral polysiloxanes are known to have even better properties due to the unique chemical structures of the polyhedral polysiloxanes, such as greater heat resistance, greater light resistance, greater chemical stability, and lower dielectric properties.
Applications of polyhedral polysiloxanes have been proposed, and some of them are intended as encapsulants for optical semiconductor devices. For example, Patent Literature 1 discloses a polyhedral polysiloxane composition containing a polyhedral polysiloxane resin having at least two oxetanyl groups, an aliphatic hydrocarbon having at least one epoxy group, and a cationic polymerization initiator. This material has a high refractive index and high light extraction efficiency. However, the polysiloxane composition of Patent Literature 1 has problems attributed to the oxetanyl and epoxy groups, such as low heat resistance and low light resistance.
In order to deal with these problems, Patent Literature 2, for example, uses an epoxy group-containing polyorganopolysiloxane with the limited glass transition temperature to improve the problems in heat resistance and light resistance. This material is also thought to be more resistant to cracking even after a thermal shock test.
However, it is still difficult to use this material in applications requiring high heat resistance and high light resistance (e.g. white LEDs), and its thermal shock resistance is not high enough.
Additionally, despite the above excellent properties, polysiloxane compositions generally have the problem of low gas-barrier properties. Unfortunately, because of this problem, these compositions, when used as encapsulants for optical semiconductor devices, may allow sulfides to turn reflectors black. An exemplary strategy to deal with this problem is to coat metal members with an acrylic resin having high gas-barrier properties in advance before encapsulating them with a silicone resin as disclosed in Patent Literature 3.
However, the silicone resin used in this technique itself has low gas-barrier properties, and additionally, this technique is problematic in terms of productivity because it requires extra steps such as encapsulation with a silicone resin separately after the coating treatment with an acrylic resin.
In the field of encapsulants for optical semiconductor devices, for example, the following techniques are employed: encapsulants containing a yellow phosphor are used for blue light emitting devices in order to produce white light; or encapsulants containing green and red phosphors are used for blue light emitting devices in order to improve color rendition. When these encapsulants have low viscosity, the phosphors may settle during the handling of the encapsulants to cause the problem of variation in emission color.
For example, Patent Literature 4 discloses a composition containing a modified polyhedral polysiloxane which has excellent moldability/processability, transparency, heat and light resistance, and adhesion. However, this material still leaves room for improvement in terms of the viscosity (handleability) of the composition and gas-barrier properties.
As described above, there is a need to develop materials that have high heat resistance and high light resistance, are excellent in thermal shock resistance and gas-barrier properties, and exhibit good handleability when used to encapsulate an optical semiconductor device.